Codec Concatenation

This was a post I main on the cinematography mailing list, I though it would be approxiate to reproduce here.

They were discussing the use of compression upon ingest, somewhat generated by our HDCAM SR vs CineForm vs uncompressed Viper tests. The discussion moved on from acquisition to post issues of compression concatenation. It seems that most now accept compression at the ingest stage as long as an uncompressed/lossless workflow follows. Certainly I would agree the acquisition compression is so light it is not going to impact the final output -- see the first posting on the results for the HDCAM SR vs CineForm tests.

Let's examine compression concatenation. We understand that concatenation between differing codecs is worse than encoding to the same codec. When compressing to the same codec as your source, a well designed codec will settle, high frequency quantization can't lose the same data twice. If the first generation is good, the subsequent generations will also be good.

Using a frame from the outdoor sequence
Standard quality first generation SR 49.48dB
Standard quality first generation CineForm 50.53dB (higher number is better)

PSNR from gen1 to gen2
Standard quality first generation CineForm 68.97dB (it settled)
The difference from source to second generation 50.48dB

So only a 0.05dB difference after an additional generation. It will take well over 10 generations for the PSNR of CineForm to be equivalent of the first generation HDCAM-SR. Yes this is a good argument for sticking with one post ready codec through-out your workflow. We experienced this three years ago when working on Dust to Glory with an early beta version of the CineForm codec; the post went through 6 or 7 generations before film-out, it still looked beautiful. In this project, much of the source was HDCAM (as well as 16 and 35mm,) so there was also a cross codec concatenation that didn't impact the final results. The reason for this is headroom. Codecs designed for post are not trying to encode the image at the smallest data rate possible, where they fall apart on the second of third generation, instead there is quality to spare to handle concatenation issues.

Here is want happens from SR to CineForm codec concatenation.
Uncompressed to SR - 49.48dB
Uncompressed to CineForm FS1 - 50.53dB
Uncompressed to CineForm FS2 - 53.10dB
SR to CineForm FS1 - 50.54dB
SR to CineForm FS2 - 53.13dB
Uncompressed to SR to CineForm FS1 - 47.12
Uncompressed to SR to CineForm FS2 - 48.02

First note that SR to CineForm is the same distortion as Uncompressed to CineForm, this means that nothing nasty is happening with the source being SR compressed, same goes for output from a CineForm project to SR. Now the cross codec conversion means there is no settling, resulting in a 1 to 2dB loss (depending on encoding mode) when going from uncompressed to SR to CineForm, further generations to the same codec will settle to an approximate 0.05dB loss per generation. Using the lowest number 47dB PSNR, let's see if that is sufficient quality. When compared to the frame to frame PSNR of 40dB, that still means we have very approximately 7dB of headroom (yes that logic will drive the image scientist nuts -- yet this is also from years of testing.) If you going between wavelet and SR five or mores time you might see issues, but otherwise you are safe. Anyone using an IT workflow will never have such cross codec issues.

Again lossless/uncompressed is the most robust workflow, but dismissing lossy compression without a fully understanding how little loss there is, could be throwing money away.